This invention relates to a valve train for an internal combustion engine and more particularly to a valve train that permits adjustment of both the valve events and the valve duration.
As is well known, many features in conjunction with the design of internal combustion engines, and particularly those for automotive vehicles, are dictated by compromises in running characteristics. The design of the valve train, and specifically the valve timing, is one of the best known of these design compromises. In order to provide adequate breathing at high engine speeds, it is desirable to provide both a long duration of the valve opening and a substantial degree of overlap. That is, in order to assure good and consistent gas flow at high speeds, it has been the practice to provide a considerable overlap between the opening of the intake valve and the closing of the exhaust valve. Such overlap is necessary to permit the induction of a full charge and to insure good scavenging of the chamber at the completion of each power stroke. On the other hand, such long duration valve opening and considerable overlap significantly reduces performance at low speeds and can increase undesirably the exhaust gas emissions. At low speeds the greater duration and longer overlap has a tendency to cause exhaust gases to backflow into the intake system and thus reduce charging efficiency. This results in the characteristic of most engines employing radical valve timing and of having extremely rough idle operation and/or the necessity for employing high idle speeds.
From the aforenoted description it should be readily apparent that engines embodying fixed valve timing must employ a compromised timing arrangement. The degree of compromise depends upon which running characteristic and engine operating speed at which the design is to be optimized.
Various arrangements have been proposed to permit variations in valve timing during engine running. In this way the valve timing may be altered to suit the specific running condition and offer improved performance and emission control over wider engine speed ranges. The variable valve timing arrangements previously known have, however, only afforded the opportunity to either advance or retard the timing events or either one set of valves (intake or exhaust) or all valves if they are all operated by the same camshaft. In essence, most of these devices have actually altered the angular relationshp between the camshaft and the crankshaft. The inventor is unaware of any arrangement that has permitted individual adjustment between the intake and exhaust valve timing when all valves are driven by a single fixed camshaft. Also, the inventor is unaware of any arrangement which permits changing of the duration of opening of the valves of an internal combustion engine through operation of the cam follower.
It is, therefore, a principal object of this invention to provide an arrangement wherein the valve timing and also valve duration may be modified during running of the engine.
It is also an object of this invention to provide an arrangement wherein the timing of the intake and exhaust valves may be controlled independently even though both valves are operated by a single camshaft.
It is a further object of this invention to provide a valve train for an internal combustion engine in which all events and valves may be adjusted during running of the engine.